The research proposed here employs single-unit recording of "head- direction" (HD) neurons in the anterodersal thalamic nuclei and post- subiculum of freely moving rats. These cells selectively discharge when the rat's head is oriented in a particular direction with respect to landmark (or allocentric) cues in the environment. It is believed that head-direction cells play a fundamental role in spatial navigation and the acquisition and maintenance of central representations of space. These abilities have been shown to be compromised in individuals with Alzheimer's disease, as well as astronauts in zero gravity. Spatial navigation is also influenced by internal idiothetic systems (i.e., vestibular, proprioceptive, optic flow). The experiments in this proposal will examine the nature of allocentric and idiothetic cue control from head-direction cells by 1) evaluating any navigational impairments in visual and non-visual versions of a food carrying task as the result of lesions in the HD circuit, 2) recording the electrophysiological responses of HD cells while rats perfo4rm the food carrying task and correlating the neuronal responses with accurate homing behavior, and 3) determining how lesions of the entorhinal cortex affect the accuracy with which the HD cell preferred firing direction shifts with a displaced landmark cue. Together, these experiments will provide a further understanding of the neural bases of representations of space and how they might guide one's ability to navigate through space.